Metal-Alkene Complexes

Experimental Structures

• Experimental structures of metal-alkene complexes can fall between two extremes: Dewar-Chatt-Duncanson (D-C) model and Metal-Centered (MCP) model.
• The D-C model fits best for intermediate oxidation state late metals, such as Zeise's salt, while the MCP model applies for Pt(0).
• Both models are considered η2 structures.

Metal-Alkene Bonding

• The alkene C=C bond length increases on binding due to two reasons:
  • M-alkene σ bond depletes the C=C π bond by donation to M, slightly weakening and lengthening the bond.
  • Back donation from the metal lowers the alkene C-C bond order by filling C=C π*.
• For weakly π-basic Pt(II), the D-C model results in a slight reduction in bond length, while for strongly π-basic Pt(0), the MCP model applies, leading to a longer C=C bond length.

η2 Complexes

• Electron-withdrawing substituents on carbon encourage back donation and strengthen the M-alkene bond.
• The bonds to the four substituents of the alkene are bent away from the metal to a small extent in the D-C case, but to a much bigger extent in an MCP complex.
• The C1 and C4 substituents twist 20-30° out of the plane of the ligand and bend back to overlap better with the metal.

Diene Complexes

• The butadiene frontier orbitals, ψ2 (HOMO) and ψ3 (LUMO), dominate bonding to the metal.
• The MO diagram shows that both depletion of electron density in ψ2 by σ donation to the metal and population of ψ3 by back donation from the metal lengthen C1-C2 and shorten C2-C3.
• Protonation can occur at C1 where the HOMO, ψ2, has its highest coefficient.

Isolobal Replacement and Metallocycles

• The chemical character of a fragment depends on the symmetry and electron occupation of the frontier orbitals.
• Fragments that are very dissimilar in composition can have similar frontier orbitals, known as isolobal fragments.
• Isolobality helps in understanding metallabenzenes, in which we replace one CH of benzene by a metal fragment isolobal with CH.

η8 Ligands

• The antiaromatic 8π electron, nonplanar hydrocarbon, η8-cyclooctatetraene (cot), can form complexes as the reduced, aromatic 10πe cot2− dianion.
• Early metals that need many electrons to achieve an 18e structure can also give η8-C8H8 complexes.

Cyclopentadienyl Complexes

• The η1-Cp group can be fluxional, in which case the metal rapidly moves around the ring to make all the protons equivalent.
• The MO scheme for M-C5H5 shows that the five carbon p orbitals lead to five MOs for the C5H5 group.
• The most important overlaps are ψ1 with the metal dz2, and ψ2 and ψ3 with the dxz and dyz orbitals.

Metallocenes

• The MO diagram for a Cp2M metallocene requires consideration of both Cp groups.
• The bonding scheme retains both the L→M direct donation and the M→L back donation, as well as a d-orbital splitting pattern that broadly resembles the two-above-three pattern characteristic of an octahedral crystal field.
• The MCp2 unit is intrinsically stable, and the same structure is adopted for numerous first-row transition metals, even when this results in a paramagnetic, non-18e complex.